We describe the science motivation and development of a pair production telescope for medium-energy (~5-200 MeV) gamma-ray polarimetry. Our instrument concept, the Advanced Energetic Pair Telescope (AdEPT), takes advantage of the Three-Dimensional Track Imager, a low-density gaseous time projection chamber, to achieve angular resolution within a factor of two of the pair production kinematics limit (~0.6° at 70 MeV), continuum sensitivity comparable with the Fermi-LAT front detector (<3 × 10-6 MeV cm-2 s-1 at 70 MeV), and minimum detectable polarization less than 10% for a 10 mCrab source in 106 s.
